1. Field of the Invention
This invention relates to a novel oxygen generating electrode. More particularly, it relates to an oxygen generating electrode suitable for use as an anode in electrolysis of a desired aqueous solution for generating oxygen at the anode and featuring improved durability and low oxygen overvoltage.
2. Prior Art
Metal electrodes in the form of conductive substrates of metallic titanium having coatings of platinum group metals or oxides thereof were conventionally used in various areas of the electrolysis industry. For example, electrodes in the form of titanium substrate coated with ruthenium and titanium oxides or ruthenium and tin oxides are known as effective anodes for generating oxygen through salt electrolysis as disclosed in Japanese Patent Publication (JP-B) Nos. 21884/1971, 3954/1973 and 11330/1975.
In the electrolysis industry, some electrolysis processes are accompanied by chlorine generation as in the case of salt electrolysis and some are accompanied by oxygen generation as in the case of acid, alkali or salt recovery, collection of metals such as copper and zinc, electrodeposition, and cathodic corrosion prevention.
If conventional electrodes for normal use in chlorine generating situations such as the above-mentioned electrodes in the form of titanium substrates coated with ruthenium and titanium oxides or ruthenium and tin oxides were used in electrolysis with concomitant oxygen generation, the electrodes could be corroded and cease to be effective within a short time. Then those electrodes specially designed for oxygen generation were used in such applications. Although iridium oxide-platinum system electrodes, iridium oxide-tin oxide system electrodes, and platinum-coated titanium electrodes are known, lead system electrodes and soluble zinc electrodes are most commonly utilized.
However, these known electrodes suffer from several troubles in particular applications and are thus not fully satisfactory. In the case of zinc electrodeposition, for example, soluble zinc anodes are so quickly dissolved that the electrode distance must be frequently adjusted. Insoluble lead anodes would produce defective deposits due to the influence of lead introduced into the electrolyte solution. Platinum-coated titanium electrodes cannot be applied to high-speed zinc plating with a high current density of at least 100 A/dm.sup.2 because of substantial consumption.
Therefore, it is one of important tasks in the electrode manufacturing technology to develop an electrode for use in electrolysis with concomitant oxygen generation which is universally applicable to a wide variety of applications without any inconvenience.
In general, when electrolysis with concomitant oxygen generation is carried out using a titanium base electrode having a coating layer as the anode, a titanium oxide layer is formed between the base and the coating layer and the anode potential gradually increases, often resulting in stripping of the coating layer and passivation of the anode. In order to prevent formation of intervening titanium oxide, passivation of the anode, and increase of electric resistance, intermediate layers are previously formed from various metal oxides as disclosed in JP-B 21232/1985, JP-B 22075/1985, Japanese Patent Application Kokai (JP-A) Nos. 116786/1982 and 184690/1985. These intermediate layers, however, are generally less conductive than the coating layers and thus, they are not so effective as expected especially in electrolysis at a high current density.
Also, JP-A 184691/1985 discloses an intermediate layer having platinum dispersed in base metal oxide and JP-A 73193/1982 discloses an intermediate layer of valve metal oxide and noble metal. The former intermediate layer was less effective since platinum is less corrosion resistant by itself. The intermediate layer having valve metal oxide mixed was difficult to achieve the desired effect since the type and amount of valve metal were naturally limited.
Also known are electrodes having a lead dioxide coating formed on a conductive metal substrate via an intermediate layer of iridium oxide and tantalum oxide (see JP-A 123388/1981 and 123389/1981). This intermediate layer is effective only for improving the adhesion between the metal substrate and the lead dioxide coating and preventing any corrosion by pinholes or defects, but not fully effective in suppressing formation of titanium oxide when used in electrolysis with concomitant oxygen generation. Additionally contamination of the electrolytic solution with lead is unavoidable.
Other known electrodes are iridium oxide/tantalum oxide coated electrodes including one having on a conductive metal substrate an intermediate layer of iridium oxide and tantalum oxide and an overcoat layer of iridium oxide (see JP-A 235493/1988) and one of the same arrangement, but having increased contents of iridium oxide in the overcoat layer (see JP-A 61083/1990 and 193889/1991). More particularly, in JP-A 61083/1990, the undercoat layer contains 2.6 to 8.1 mol% of Ir and the overcoat layer contains 17.6 to 66.7 mol% of Ir while there is shown a comparative example having an undercoat layer with 16.7 mol% Ir. In JP-A 193889/1991, the undercoat layer contains 40 to 79 mol% of Ir (30 mol% in a comparative example) and the overcoat layer contains 80 to 99.9 mol% of Ir. Therefore known undercoat layers which are Ir poorer than the overcoat layer have Ir contents of up to 8.1 mol% or at least 16.7 mol%. Power losses occur since the iridium oxide in the overcoat layer has a higher oxygen overvoltage than the intermediate layer of iridium oxide and tantalum oxide. These electrodes are unsatisfactory in change with time of oxygen overvoltage after electrolysis and short in lifetime. A bond strength lowering at the end of electrolysis is also a problem.
JP-B 55558/1991 discloses a single iridium oxide-tantalum oxide coating with an Ir content of 19.8 to 39.6 mol%. This electrode is also unsatisfactory in oxygen overvoltage, lifetime and bond strength.
Electrodes having a low oxygen overvoltage are also known. For example, JP-A 301876/1989 discloses an electrode having a coating of iridium oxide, tantalum oxide and platinum. This electrode is expensive since iridium and platinum must be used in the undercoat layer. It is less advantageous in lifetime and degradation with time than the iridium oxide/tantalum oxide coated electrodes. A bond strength lowering at the end of electrolysis is also a problem.
Also known are electrodes having a dispersion coated intermediate layer of platinum and iridium oxide or base metal oxide and an overcoat layer of iridium oxide or platinum and valve metal oxide (JP-A 190491/1990, 200790/1990, and 150091/1984). These electrodes, however, are not so long lasting as expected and the intermediate layer is costly.
JP-A 294494/1990 discloses an electrode having an intermediate layer of platinum or iridium oxide and valve metal oxide and an overcoat layer of platinum or lead dioxide, which has a high oxygen overvoltage and a relatively short lifetime.